Oilfield drilling fluid, often called “mud,” serves multiple purposes in the industry. Among its many functions, the drilling mud acts as a lubricant to cool rotary drill bits and facilitate faster cutting rates. Typically, the mud is mixed at the surface and pumped downhole at high pressure to the drill bit through a bore of the drillstring. Once the mud reaches the drill bit, it exits through various nozzles and ports where it lubricates and cools the drill bit. After exiting through the nozzles, the “spent” fluid returns to the surface through an annulus formed between the drillstring and the drilled wellbore.
Furthermore, drilling mud provides a column of hydrostatic pressure, or head, to prevent “blow out” of the well being drilled. This hydrostatic pressure offsets formation pressures thereby preventing fluids from blowing out if pressurized deposits in the formation are breeched. Two factors contributing to the hydrostatic pressure of the drilling mud column are the height (or depth) of the column (i.e. the vertical distance from the surface to the bottom of the wellbore) itself and the density (or its inverse, specific gravity) of the fluid used. Depending on the type and construction of the formation to be drilled, various weighting and lubrication agents are mixed into the drilling mud to obtain the right mixture. Typically, drilling mud weight is reported in “pounds,” short for pounds per gallon. Generally, increasing the amount of weighting agent solute dissolved in the mud base will create a heavier drilling mud. Drilling mud that is too light may not protect the formation from blow outs, and drilling mud that is too heavy may over invade the formation. Therefore, much time and consideration is spent to ensure the mud mixture is optimal. Because the mud evaluation and mixture process is time consuming and expensive, drillers and service companies prefer to reclaim the returned drilling mud and recycle it for continued use.
Another significant purpose of the drilling mud is to carry the cuttings away from the drill bit at the bottom of the borehole to the surface. As a drill bit pulverizes or scrapes the rock formation at the bottom of the borehole, small pieces of solid material are left behind. The drilling fluid exiting the nozzles at the bit acts to stir-up and carry the solid particles of rock and formation to the surface within the annulus between the drillstring and the borehole. Therefore, the fluid exiting the borehole from the annulus is a slurry of formation cuttings in drilling mud. Before the mud can be recycled and re-pumped down through nozzles of the drill bit, the cutting particulates must be removed.
Apparatus in use today to remove cuttings and other solid particulates from drilling mud are commonly referred to in the industry as “shale shakers.” A shale shaker is a vibrating sieve-like table upon which returning dirty drilling mud is deposited and through which clean drilling mud emerges. Typically, the shale shaker is an angled table with a generally perforated filter screen bottom. Returning drilling mud is deposited at the top and as the drilling mud travels down the incline toward the lower end, the fluid falls through the perforations to a reservoir below leaving the solid particulate material behind. The combination of the angle of inclination with the vibrating action of the shale shaker table enables the solid particles left behind to flow until they fall off the lower end of the shaker table. Preferably, the amount of vibration and the angle of inclination of the shale shaker table are adjustable to accommodate various drilling mud flow rates and particulate percentages in the drilling mud. After the fluid passes through the perforated bottom of the shale shaker, it can either return to service in the borehole immediately, be stored for measurement and evaluation, or it may pass through an additional piece of equipment (e.g. a drying shaker, centrifuge, or a smaller sized shale shaker) to further remove smaller cuttings.
Because shale shakers are typically in continuous use, any repair operations and associated downtimes are to be minimized as much as possible. Often, the filter screens of shale shakers, through which the solids are separated from the drilling mud, wear out over time and need replacement. Therefore, shale shaker filter screens are typically constructed to be quickly and easily removed and replaced. Generally, through the loosening of only a few bolts, the filter screen can be lifted out of the shaker assembly and replaced within a matter of minutes. While there are numerous styles and sizes of filter screens, they generally follow the same design. Typically, filter screens comprise a perforated plate base upon which a wire mesh, or other perforated filter overlay, is positioned. The perforated plate base generally provides structural support and allows the passage of fluids therethrough while the wire mesh overlay defines the largest solid particle capable of passing therethrough. While many perforated plate bases are generally flat or slightly curved in shape, it should be understood that perforated plate bases having a plurality of corrugated, or pyramid-shaped channels extending thereacross may be used instead. In theory, the pyramid-shaped channels provide additional surface area for the fluid-solid separation process to take place and act to guide solids along their length toward the end of the shale shaker where they are disposed of.
A typical shale shaker filter screen includes a plurality of hold-down apertures at opposite ends of the filter screen. These apertures, preferably located at the ends of the filter screen that will abut walls of the shale shaker, allow hold down retainers of the shale shaker to grip and secure the filter screens in place. However, because of their proximity to the working surface of the filter screen, the hold-down apertures must be covered to prevent solids in the returning drilling fluid from bypassing the filter mesh through the hold-down apertures. To prevent such bypass, an end cap assembly is placed over each end of the filter screen to cover the hold-down apertures. Presently, these caps are constructed by extending a metal cover over the hold down apertures and attaching a wiper seal thereto to contact an adjacent wall of the shale shaker. Furthermore, epoxy plugs are set in each end of the end cap to prevent fluids from communicating with the hold-down apertures through the sides of the end cap.
While current filter screen end caps are effective, they are complex structures that take considerable time and expense to fabricate. A less expensive and complicated end cap would be well received in the industry.